Apply BFG Object map might fail due to race condition

The Projects::CleanupService calls the GitGarbageCollectWorker without a lease key. This results in a race condition with any other call to GitGarbageCollectWorker without a lease key because the exclusive key being used on Redis is: "gitlab:exclusive_lease:#{key}" and the key is nil.

GitGarbageCollectWorker should always require a lease_key, or maybe default to "#{task}:#{project_id}?

/cc @nick.thomas